iPad sales have been flagging lately, but this past week they were given a boost from Apple’s two biggest rivals. Google released the unsatisfying Nexus 9, which had posed the biggest threat of dethroning the iPad, and Microsoft made its Office suite of apps completely and utterly free on iOS. Without Apple moving a muscle, the iPad is today smarter, more productive, and apparently better value than it was just a few days ago. Such has been the history of this device since its inception.

The original iPad, now approaching the grand old age of 5, was announced with a quaintly small selection of "12 next-generation Multi-Touch applications." Apple laid down an excellent foundation of well designed hardware and responsive software and then waited for others to complete the job. Buoyed by the preceding success of the iPhone and profitability of the App Store, developers flocked to Apple’s latest creation and quickly built up the enormous software ecosystem that it now enjoys as its biggest advantage. As a device, the iPad is vulnerable to the caprices of an evolving and increasingly saturated market. As a platform for a new type of mobile computing, however, it now appears unassailable.

AS A DEVICE, THE IPAD'S VULNERABLE, BUT AS A PLATFORM, IT'S ALMOST UNASSAILABLE

Google’s Nexus 9 is, ironically enough, the best illustration of what makes the iPad so compelling. The new flagship Android tablet moves to the same 4:3 screen ratio as the iPad and feels more natural and appropriate to its 9-inch size than the typical widescreen display. Android Lollipop is also Google’s best and most refined tablet software to date, though it’s only setting the stage for an exhibition that never comes. The constellation of apps that illuminates the iPad user experience and makes people so attached to it is absent from the Android alternative. Even as Apple’s own improvements to the iPad’s software stagnate and Google makes strides to catch up, the halo of active third-party app developers ensures that the iPad remains supreme.In spite of its brief history, the iPad has already played host to a number of standout apps that have raised expectations for all mobile devices. The Paper app from FiftyThreeis still the prettiest sketchbook out there, Flipboard got its start on the iPad, and the iOS-exclusive Infinity Blade continues to set new standards for graphics quality in mobile games. Apps like Traktor DJ and Auxy are also turning the iPad into a legitimate music-making instrument. And now Microsoft is delivering its best mobile Office experience on Apple’s devices.

Here’s Office for iPhone vs. Office for Windows Phone with the same document open. Guess which is better? iPhone pic.twitter.com/NfnCeIDipz

Revolutions are characterized by sudden, often violent change that would have been difficult to predict in advance. That’s what is happening at Microsoft right now, as new CEO Satya Nadella is remolding the company around a vision of ubiquity on every platform. The choice to make Office free on iOS and Android is informed by the desire to maintain Word, Excel, and PowerPoint as industry standards, and the biggest target of this push is Apple’s iPad. Originally derided as just a big iPod touch, the iPad has been quietly carving a space for itself as a competent business machine that can serve as anything from an estate agent's portfolio, to a journalist's notebook, to a DJ’s sidekick.

There’s a complex web of causation that lies behind Microsoft’s actions — one that includes Google’s cloud-based Docs and Android’s meteoric rise as the dominant mobile OS — but the upshot for Apple is simply that the iPad’s popularity is being fed by the companies that are supposed to try and beat it. Microsoft made a sincere effort to compete with the iPad through its Surface RT tablet, but it crashed against the same unforgiving barrier of app inadequacy as every Android tablet to date. This year’s Surface Pro 3 is "the tablet that can replace your laptop," more concerned with defeating the MacBook than challenging the iPad. Amazon similarly dodges direct confrontation with the iPad by making its Kindle Fire tablets into cheap consumption devices that make their money through Amazon-sold content. Like Microsoft, Amazon prefers to have its services on Apple’s platform over trying to take it down from the outside.

The Nexus 9, Samsung’s Galaxy Tab S, and Sony’s Xperia Z3 Tablet Compact are the most recent contenders to have dared to directly challenge the iPad, but all they’ve done is underscore the magnitude of Apple’s lead. Even at $100 less than Apple’s 9.7-inch Air 2, the Nexus 9 seems like a bad deal when compared to the refined design, excellent battery life, and multipurpose versatility of the iPad. As if to emphasize that point, HTC had the Nexus 9 on fire sale the day after its release. Summoning up the combined engineering and financial might of Google and HTC has produced yet another device destined to labor in the iPad’s looming shadow.This is to be expected. Outside of the iPad, there’s very little evidence that making tablets is a profitable or even viable business. Back in 2010, the hastily assembled HP Slate preempted the original iPad announcement by a few weeks, but flopped terribly. A year later, the BlackBerry PlayBook, the Motorola Xoom, and the webOS-powered HP TouchPad again proved that making a commercially viable large-screen tablet was hard. Meanwhile, the iPad was selling about as fast as Apple could make it, surpassing 3 million sales per month by the middle of 2011.

THE IPAD'S GREATEST ACHIEVEMENT HAS BEEN SIMPLY TO CONVINCE PEOPLE TO USE IT

The iPad still doesn’t have a clearly defined reason for its existence. Apple never bothered to give it one, focusing instead on the engineering and trusting that users will figure out ways to adapt it to their lives. As Apple design chief Jony Ive puts it, "I don’t have to change myself to fit the product; it fits me." Time has proven this philosophy correct, as the iPad has risen above its physical limitations and secured itself a niche in hundreds of millions of homes.The iPad’s debut was greeted with skepticism, derision about it merely reiterating the iPhone at a larger size, and a roll of the eyes for its "magical and revolutionary" tagline. Four years on, it has earned a level of ubiquity and name recognition approaching the likes of Xerox and Kleenex, with NFL commentators describing Microsoft’s Surface Pro tablets as "iPad-like tools" and CNN talking heads using Surfaces as iPad kickstands.Perhaps a big iPhone wasn’t such a bad idea after all.